Missing traces from akka connection-pool apis

[Marcus-Rosti]

per: https://github.com/DataDog/dd-trace-java/issues/2157#issue-756199296

The java agent is missing traces from anything that uses any of the connection pool apis: https://doc.akka.io/docs/akka-http/current/client-side/host-level.html https://doc.akka.io/docs/akka-http/current/client-side/connection-level.html https://doc.akka.io/docs/akka-http/current/client-side/request-level.html#flow-based-variant